To do that in the Twenty Eleven theme, you'll want to zero out the left and right margin for the "content" and "secondary" IDs. If you do that, you'll probably also want to adjust the width of the content area too. Start with this CSS to adjust pages that have a sidebar:

When you say "the limit of the page" are you referring to the entire browser width? If so, keep in mind that different people will have different monitor sizes and the margins will look different depending on each setup. You could get rid of the size restriction of the "#page" element, but personally I think there's a better chance it will look good if you stick with the current layout. If you want to make "#page" full-width anyway, here's how to do it:

That code removes the header's max width of 1000 pixels, but there's always a margins between the headers's sides and the browser window frame (let's call it that).

What I was trying to do was having the header and footer occupy the full width of the page, with no margins (as I was able to do at the top, for instance, where the header is now touching the edge of the page without a margin)